Ostwald's rule of stages
In the traditional model of crystallization, atoms or molecules nucleate into clusters, and the clusters rapidly grow after reaching a critical radius. However, the system does not necessarily transform directly into the most energetically favorable state. That is, there might be intermediate stable states as a system transforms into its ultimate crystalline state. This was predicted by Ostwald in 1897, and is thus called Ostwald's rule of stages.
Chung, S.-Y., Kim, Y.-M., Kim, J.-G. & Kim, Y.-J., Nature Phys. 5, 68-73 (2009).